Xv is "The XFree86 X Video Extension (Xv) Device Dependent Layer", an extension to XFree86 to allow for hardware accelerated video playback.

It was introduced in XFree86 4. (See the Xv(3x) man page.)

To find out if you're using it, try xvinfo(1). xvattr is also useful to set parameters for it.

To use it, add

Load "v4l"

in the module section of your /etc/X11/XF86Config file (for XFree86 version 4) or /etc/X11/xorg.conf (if using the X.org XServer). This allows the "Xvideo" extension to do video overlays and scaling.

One useful effect of this is that some tv viewing apps don't need to send those annoying refresh commands that make your desktop flicker every time the window is resized, or another window obscures or reveals part of its window.